The present invention relates to ink jet recorders and more particularly to an ink jet recorder having an improved means for catching the portion of the ink drops which are not intended to strike the print web. Ink jet recorders are shown generally in U.S. Pat. No. 3,373,437, issued Mar. 12, 1968 to Sweet et al; U.S. Pat No. 3,586,907 issued June 22, 1971 to Beam et al; and U.S. Pat. No. 3,701,998 issued Oct. 31, 1972 to Mathis.
The ink jets in such recorders are created by forcing a supply of recording fluid from a manifold through a series of orifices in an orifice plate. The orifice plate is mechanically stimulated so that the jets break up into streams of uniformly sized and regularly spaced drops. Each stream of drops is formed in proximity to an associated charging electrode which induces electrical charges in selected ones of the drops as they are formed. The streams of drops then fall toward the moving print web.
Only selected ones of the drops are intended to strike the web, however, and an electrical deflection field is provided in the path of the drops in order to cause the charged and uncharged drops to diverge into two separate trajectories. The drops in one set of trajectories may be caught by suitable apparatus and, therefore, prevented from printing. In order to conserve on the amount of ink used in the printing process, the caught drops are filtered and then recirculated back to an ink reservoir for later use in printing. The drops in the other trajectory are not caught, however, and therefore strike the print web. Printing with charged drops requires precise deflection of the drops to their ultimate print positions on the print web, and therefore drop size and the charge level on the drops are critical. A number of prior art systems have avoided these problems to an extent by using the uncharged drops for printing, while catching and recirculating the charged drops.
It will be appreciated that the means by which the deflected drops are caught and prevented from striking the print web is critical in the operation of the printer. A number of approaches have been taken to insure that the drops are effectively caught.
In U.S. Pat. No. 3,373,437 to Sweet et al, issued Mar. 12, 1968, a catcher mechanism is shown in which a porous catcher surface is positioned parallel to the drop streams. Deflected drops strike the porous surface and are ingested into a central cavity which is maintained under at least a partial vacuum. One difficulty with such a porous catcher is that drops striking the porous surface tend to splatter with the result that droplets of ink may strike the web and degrade the quality of the printed image.
Catchers of the type shown in U.S. Pat. No. 3,701,998 to Mathis, issued Oct. 31, 1972 have been somewhat more successful. The catchers used in the Mathis device each have a smooth, vertical catching surface formed of a nonporous material. A slot adjacent to the bottom edge of the catching surface provides an access to an interior, pressureevacuated cavity into which the drops are ingested, after striking the catching surface.
Although functioning in a manner superior to the catchers of Sweet et al, supra, the Mathis catchers are somewhat disadvantageous in that the air flow through the ink ingesting slot may cause air turbulence in the path of the undeflected drops and thus alter the points at which the drops strike the print web. This turbulence may also attract dirt particles which may lodge between the catcher and an adjacent deflection field electrode, thus shorting out the electrode.
Additionally this type of catcher has a relatively large vertical dimension and, as a result, the jet orifices and charge rings are constrained to be positioned well above the moving web. Although the jet orifices are manufactured to exacting tolerances, it will be appreciated that it is not possible to make these orifices precisely vertical. As a result, there will necessarily be a slight skew in many of the jets such that the drops formed from these jets will not strike the web at precisely the desired position. The further orifices are positioned from the print web, the more noticable these errors will become. Other catchers having similar limitations are shown in U.S. Pat. Nos. 3,836,914, issued Sept. 17, 1974 to Duffield; and U.S. Pat. No. 3,813,675, issued May 28, 1974 to Steffy et al.
Another type of catcher arrangement is shown in U.S. Pat. No. 3,936,135, issued Feb. 3, 1976 to Duffield. This catcher mechanism is generally similar to the Mathis catcher described above. The central cavity in this catcher is, however, filled with conductive ink which is removed at a rate approximating that at which the ink is deposited on the catching surface. The result of this is that the ink completely fills the ingesting slot and forms a miniscus at that point. There is no air flow into the catcher and, therefore, air disturbances and the resulting print image degradation are minimized. This catcher configuration is, however, capable of handling only moderate ink flow rates. Additionally this catcher, like many prior art catchers, accentuates the errors resulting from orifice skew.
A need, therefore, exists for a relatively thin catcher mechanism which will allow the jet forming orifices to be positioned near the moving print web and which will create only a minimal air disturbance in the path of the drops to be printed.